1. Field of the Invention
This invention relates to polyester films suitable for use as substrates for magnetic recording media such as magnetic video tapes, magnetic video discs and floppy discs. More specifically, this invention relates to smooth and slippery polyester films having excellent performance in electromagnetic characteristics far superior to conventional polyester films and thus suitable for use as substrates for magnetic recording media as well as polyester films capable of permitting recording at still higher densities, excellent in travelling performance and suitable for use as substrates for magnetic recording media.
2. Description of the Prior Art
There has been made in recent years a striking achievement in the increase of recording density of magnetic recording media and efforts are still continued to allow recording at still higher densities.
Improvements have been made to magnetic materials to increase their recording densities. For example, the magnetic material has been improved from .gamma.-Fe.sub.2 O.sub.3 which is used in general audio tapes, floppy discs and the like, to CrO.sub.2 having higher performance, and further to magnetic powder consisting mainly of pure iron which magnetic powder is commonly called the metallic magnetic material. Accompanied by such improvements to magnetic materials, developments of improved magnetic recording media are now under way including the so-called metal-coated magnetic recording media bearing magnetic layers, which contain pure metal as a principal component, applied on the surfaces thereof and magnetic recording media carrying metal thin layers containing no extra medium therein and formed 100% of ferromagnetic metal, which film layers have been formed by metal deposition on the surfaces of the magnetic recording media.
Biaxially oriented and heat-set polyethylene terephthalate films have been used as base films for high-density recording media similar to conventional magnetic tapes, owing to their good heat resistance, excellent mechanical strength and weathering resistance, and relatively inexpensive prices.
When producing magnetic recording media by forming ferromagnetic metal thin layers on the surfaces of such polyester films by coating or metal deposition, the polyester films are required to have far better surface characteristics compared with those of polyester films conventionally employed for magnetic tapes. Namely, it is necessary to make the recording wavelengths still shorter in order to increase the recording densities of magnetic tapes. Reflecting this requirement, the magnetic layers must be made thinner. For example, it is necessary to make the thickness of a coating thinner to about one half or less compared with conventional magnetic media formed principally of .gamma.-Fe.sub.2 O.sub.3 even if the magnetic layers are applied by the coating techinque, when a magnetic material consisting principally of pure ferromagnetic metal is used. In the case of magnetic recording media bearing thin film layers of a magnetic metal applied directly on the surfaces of their bases by deposition or the like, the the magnetic layers become still thinner by one figure and must be reduced to as thin as about 0.5 .mu.m or so.
Where a magnetic layer is as thin as mentioned above, the surface roughness of its base film will lead to the surface unevenness of the resultant magnetic layer more directly compared with conventional magnetic layers. Thus, it is necessary to make the surfaces of polyester films, which are to be used in high density magnetic recording media, still smoother and more slippery compared with conventional polyester films. For example, in order to reduce the gap loss between a magnetic head and the surface of a magnetic layer to 1 dB or smaller when recording at such a short wavelength as 1 .mu.m, it is necessary that the distance between the magnetic head and magnetic layer be 200 .ANG. or shorter. As have been mentioned above, the surface smoothness of a polyester base film gives significant direct influence on electromagnetic characteristics such as video outputs, variations in output along the passage of time, omission of signals and/or outputs.
It has also been known that, even in the case of conventional magnetic tapes, one third of the surface roughness of projections of their base films, i.e., polyester films, will appear as roughness of the magnetic layers of resultant magnetic tapes. Accordingly, there is a continuing need for still smoother films having excellent and increased slipperiness. It is an essential requirement for improved electromagnetic characteristics of video tapes and for the production of high-quality magnetic tapes to make their base films, i.e., polyester films still smoother and excellent in slipperiness.
As another essential requirement for using a polyester film as the base film of a magnetic recording medium, the film surfaces are not supposed to have wrinkles or flaws. For these requirements to be satisfied the base film is required also to have, besides smoothness, good slipperiness, namely, a smaller friction factor, because a film having poor slipperiness are susceptible of causing flaws or wrinkles during its production, in the course of forming a magnetic layer by coating or deposition or otherwise handling the film. Such a film either cannot be used as a base film or, even if it should be used despite of such drawbacks, the production yield will be made extremely poor, thereby pushing up the production cost. Good slipperiness is required also after processing the film into magnetic tapes and the like for permitting smooth travelling of the magnetic tapes and the like when paying them out from reels or cassettes and winding them up on reels or cassettes. When a magnetic layer is rendered thinner to achieve high density recording as in video tapes produced by vacuum evaporation or their analogous products, surface defects such as flaws caused by such poor slipperiness, fold wrinkles, remains of tiny bubbles and the like will act as direct causes which will induce lowered electromagnetic characteristics, particularly, lowered video characteristics and increased dropout. Thus, production of magnetic tapes with a base film having poor slipperiness will cause serious deleterious effects to recording and reproduction due to lowered travelling performance.
As has been described above, a polyester film to be used as a base film for magnetic recording media is required to satisfy mutually-contradictory surface characteristics, i.e., to achieve smoothness as much as possible on one hand and to impart slipperiness as much as feasible on the other hand. Conventional magnetic tapes also need to meet such requirements to varied extents. However, the present requirements have become stringent to such high extent when compared with those required before, that the polyester film is now required to provide for further quality improvement and high density recording. In other words, the slipperiness of conventionally known polyester films will be abruptly lowered if they are rendered smooth to improve their electromagnetic characteristics. Accordingly, a coating called "back coat" is applied to the surface opposite to the magnetic surface of a polyester base film in order to impart slipperiness thereto so that the qualities of resultant magnetic tapes would be improved. However, this is cumbersome and thus disadvantageous from the industrial standpoint. In the case of vacuum-deposited metal thin-film tapes, they would become unsuitable for further application as magnetic tapes even if their surfaces opposite to their magnetic layers are rendered slippery by certain technique, unless the travelling performances between their magnetic layers and corresponding heads are improved. Therefore, it has been waited for the development of films whose slipperiness reduction remains at low levels even when their surface smoothness is increased.